Some statements may merely provide background information related to the present disclosure and may not constitute prior art.
Oil and natural gas are produced from wells having porous and permeable subterranean formations. The porosity of the formation permits the formation to store oil and gas, and the permeability of the formation permits the oil or gas fluid to move through the formation. Permeability of the formation is essential to permit oil and gas to flow to a location where it can be pumped or flowed from the well. In many cases the permeability of the formation holding the gas or oil is insufficient for economic recovery of oil and gas. In other cases, during operation of the well, the productivity of the formation drops to the extent that further recovery becomes uneconomical. In such cases, it is necessary to hydraulically fracture the formation and prop the fracture in an open condition by means of a proppant material or propping agent. Such fracturing is usually accomplished by hydraulic pressure, and the proppant material or propping agent is a particulate material, such as sand, resin coated sand or ceramic particles (all of which can be referred to as “proppant”), which are carried into the fracture by means of a fracturing fluid, typically containing high molecular weight polymers, such as guar gum, guar gum derivatives such as hydroxypropyl guar (HPG), carboxymethyl hydroxypropyl G (CMHPG), cellulose, cellulose derivatives such as hydroxyethyl cellulose (HEC), polyvinyl alcohol, biopolymers, such as xanthan gum and diutan gum, which increase the viscosity of the fracturing fluid. The fracturing fluid can also be based on a non-polymeric viscosifier as for example viscoelastic surfactant.
Crosslinking agents can also be added to the fracturing fluid to generate cross-linked gelled fluids so as provide even higher viscosities, better proppant transport properties and to create fracture geometries not possible with other types of fluids. These cross-linked gelled fluids are highly viscous but non-Newtonian and shear thinning permitting them to be easy placed. While the viscous nature of the fluids is important for proppant transport, once the proppant is placed in the fracture it is not desirable for such fluids to remain in the proppant pack as the fluids can significantly hinder the flow of oil or gas in the propped fracture. In recognition of this, the fracturing fluids include “breakers” of various types that are designed to break the chemical bonds and reduce the molecular weight of the polymeric materials in such fracturing fluids after the proppant is placed thus dramatically reducing the viscosity of the fracturing fluid and allowing it to be easily flowed back to the surface from the proppant pack. Such chemical breakers are typically added directly to the fluid. While the breakers are designed to break the chemical bonds and reduce the molecular weight of the polymeric materials in such fluids and significantly lower the viscosity of the fluids, it is important the breakers not reduce the fluid viscosity and transport capability prematurely while the fluid is being pumped. If a premature “break” of the fluid occurs during the fracturing operation, the loss of viscosity will dramatically limit the transport characteristics of the fracturing fluid. If this occurs while pumping, proppant can accumulate near the well bore rather than being carried into the created fracture. Such near well bore accumulation of proppant can lead to an early termination of a fracturing job due to excessive pumping pressure. This early termination is often referred to as a “screen out”. Conventional techniques for attempting to avoid an early breaking of the fluid viscosity have included limiting the amount of breaker added to the fracturing fluid and/or encapsulating the breaker with a material that will limit the contact of the breaker with the high molecular weight and/or cross-linked polymers in the fracturing fluid during pumping.
There are also many other applications in which breakers are needed to decrease the viscosity of treatment fluids, such as gravel packing and acidizing fluids, viscosified with polymers or crosslinked polymers or viscoelastic surfactants.
Thus, there are needs for improved compositions and methods of slow release of breaker in treatment fluids.